The present invention relates to electrostatic coating of liquid materials and more particularly to apparatus to electrostatically coat an elongated object of indefinite length.
Electrostatic spray coating has proven to be an effective method of coating a wide variety of articles. The attraction between the charged coating particles and the oppositely charged object(s) reduces overspray and thus saves coating material. The greater attraction for charged particles of portions of the surface of the object with the least amount of coating, tends to even out the application and provide a more uniform coating upon the object.
The airborne solvents, particulates or aerosols resulting from electrostatic coating with a variety of organic solvent-based paints or liquid coatings may present a toxic hazard to workers or to the greater environment and should be properly disposed of. A particular hazard of electrostatic coating with a solvent-based coating material is the possibility of explosion and/or fire. To provide a sufficient charge to the coating particles which are to be attracted to a grounded object, the electrostatic coating applicators should be charged with upward of 60,000 volts. With such voltages, arcing is an ever present likelihood. The explosive hazard occasioned by volatile particulates and organic solvents in the presence of electrical arcing or sparking has made it necessary to provide high exhaust rates to maintain a safe condition for electrostatic spraying of solvent-based coatings. The high exhaust rate of heated or cooled plant air adds to the energy expenditure of electrostatic coating.
Various solutions have been proposed and utilized to overcome inherent safety factors. To protect workers from airborne coating particles and to comply with various governmental safety regulations, electrostatic coating is commonly carried on in spraying booths or chambers wherein coating-laden air is directed away from the worker, filtered and exhausted.
Liquid coating materials may be highly conductive and may form a natural path to ground when they coat the inside of a spray chamber. To appropriately insulate a coating chamber, it may be necessary that the entire spray coating system be isolated from ground as described in U.S. Pat. No. 3,929,286.
Due to the high explosive potential of organic solvent-based electrostatic coating, alternatives such as water-based and powdered coating materials are more widely used. However, these alternatives have their own associated application problems and are not always the most suitable. Deposits of powder coating may quickly build up in a coating chamber. The use of highly conductive water-based coatings increases the inherent shock hazards. Furthermore, each coating has its own characteristics making each coating particularly suitable for specific applications. Accordingly, it is desirable that organic solvent-based coatings be included among the available coatings which may be efficiently and safely applied electrostatically.
The coating of elongated items such as wire, pipe, conduit or strips present particular problems. The length of such items necessitates that such items be passed through and extend from a chamber wherein the item is continuously coated. Because an elongated item must pass through the chamber, it is problematical to provide a sufficiently enclosed chamber to prevent contamination of the surrounding atmosphere. Furthermore, as the object extends from the spray coating chamber, the elongated object must not be highly charged so as to create an electrical shock hazard to workers. Above-mentioned U.S. Pat. No. 3,396,669 (wire) and U.S. Pat. No. 3,598,626 (pipe) describe systems in which elongated objects are passed through coating chambers and continuously electrostatically coated with powder.
It is an object of the present invention to provide apparatus whereby elongated objects of indefinite length may be continuously electrostatically coated with various liquid coatings. It is a further object to operate such apparatus in a manner in which the danger of electrical shock to users is minimized. It is a still further object to provide an electrostatic coating system in which organic solvent-based coatings may be used and in which the danger of explosion is substantially eliminated. Still further objects of the invention are to prevent overspray of coating material in the chamber from dripping onto the elongated object and to recover the overspray for reuse.